text: Many hypoxic environments never reach the level of anoxia and most fish are able to cope with this stress using different physiological and behavioural strategies.  Fish that use air breathing organs (ABO) tend to live in environments with highly variable oxygen content and rely on aerial respiration during times when there is not enough oxygen to support water-breathing.  Though all teleosts have some form of swim bladder, many of them are not capable of breathing air, and they rely on aquatic surface respiration as a supply of more oxygenated water at the surface of the water.  However, many species of teleost fish are obligate water breathers and do not display either of these surface respiratory behaviours.
  Typically, acute hypoxia causes hyperventilation, bradycardia and an elevation in gill vascular resistance in teleosts.  However, the benefit of these changes in blood pressure to oxygen uptake has not been supported in a recent study of the rainbow trout.  It is possible that the acute hypoxia response is simply a stress response, and the advantages found in early studies may only result after acclimatization to the environment.